The short answer
A roofer inspects a flat roof by walking the surface where it is safe, checking the membrane for blisters, splits and open laps, looking at the edges, upstands, flashings and outlets, testing for ponding and a soft deck, and examining the underside from the ceiling or loft for damp. Moisture readings or a thermal image may be added when the cause of a leak is not obvious.
Starting with the surface
A flat roof is usually inspected on foot, because the covering will take a person's weight and a close look is the only way to find small faults. The first task is to see the whole surface in one go, then work through it in sections.
What the roofer looks for depends on the covering. On felt, a built-up or torch-on system, the signs are blisters, cracks in the surface, splits along the laps, and a surface that has lost its mineral or solar finish. On EPDM, a synthetic rubber sheet, the concerns are punctures, shrinkage at the corners, and lifting at the adhesive joints. On GRP, a glass-fibre laminate set in resin, hairline cracks at the edges and where the deck moves are the common faults. On lead and other metals, the question is whether the sheets are expanding and splitting at the joints.
Debris counts as a finding as well. Leaves, moss and silt hold water on the roof, which shortens the life of the membrane, and they block the outlets.
Falls, ponding and the drainage
No flat roof is truly level; it is built with a gentle slope that sends water towards an outlet. BS 6229:2018 recommends a minimum finished fall of 1:80, and designing to 1:40 allows for the deck's deflection and the builder's tolerance. If puddles are still there a day or more after rain, that is ponding, and it means the fall has failed or the deck has sagged.
Ponding matters because standing water works against the weak points. It finds pinholes and seams, it carries dirt, and in winter it freezes and expands. Our page on the roof condition report explains how standing water and other findings are recorded for someone else to read.
The roofer then follows the water: into the gutter, the outlet or the hopper, and down. Gutters must be clear and joints sealed. A gutter set with no fall at all, or a downpipe that discharges at the wall, is a common cause of damp inside. On a roof with a parapet, the inspection includes the parapet gutter, a lead or metal channel behind a low wall, and the box gutter, which runs between two slopes. Outlets there are small and block easily.
Edges, upstands and flashings
The majority of leaks start at a junction, not in the middle of the roof. A good inspection spends more time at the edges than on the field of the roof.
- Upstands, the membrane turned up the face of a wall, need to be high enough, securely fixed at the top and sealed.
- Flashings, the lead or other strip that covers a joint, should be tight to the wall and not lifting.
- Drip edges and trims at the front of the roof must deliver water into the gutter rather than behind the fascia.
- Penetrations such as pipes, vents, rooflights and aerials have collars and seals that wear.
- Joints in the deck or the roof structure can open as the building moves.
The roofer presses along the upstands and laps to feel for loose areas, and checks whether sealant has cracked or pulled away. Many flat roof leaks are repairable at the edge detail, at a cost far below that of a new covering. Repair is the answer in many cases, and the survey tells you which kind of roof you have.
The deck and the underside
The deck is the structure the membrane is laid on, usually timber boards or sheets, though it may be concrete. A soft or spongy patch underfoot means the deck has absorbed water and begun to rot. That changes the decision from a surface repair to a bigger job, because the covering cannot be re-laid on a deck that will not hold it.
The roofer then looks at the roof from below. From the ceiling or the loft, damp stains, mould, dark timber and wet insulation point to a leak or to condensation. Telling the two apart is the hard part, and it is part of what the inspection is for. In a warm-deck roof the insulation lies above the deck and keeps the structure warm. A cold-deck roof has insulation between the joists, with a ventilated space above it, and a failure in that ventilation can make the deck wet from the inside.
If the cause is not clear, a damp meter, or a thermal image from a heat camera, can help. Our page on the new-build roof inspection shows the same set of checks applied to a recently completed flat roof.
What happens next, and who can do it
The result of a flat roof inspection is a short list of findings, ranked by urgency, with photographs. It might read: repair the failed seam at the north upstand, clear and reseal two outlets, monitor a shallow area of ponding. Or it might read: the membrane is spent, the deck is wet in two places, and the roof should be replaced.
Typical lifespans are a rough guide only. MyBuilder's flat roof guide (2026) gives replacement costs of about £2,800 to £7,500, and sources vary on lifespan: felt commonly 10 to 20 years, EPDM 20 to 30 years or more, GRP 25 to 40 years. Exposure, workmanship and maintenance move those numbers a good deal. Your own price, fixed and written, comes after the survey.
Not sure who should carry it out? Read roofer or surveyor for the difference. A roofer is usually the right choice when you want repairs, and a surveyor when you need an independent report for a purchase or dispute. Once a year is a sensible habit on a flat roof, and after any storm. The roof inspections page covers the other kinds of visit.
